Awwwww. I love this! I love how sweet they are together, and Harry catching up to grab Severus' arm that first day they're sightseeing. And these lines make me laugh: "And you don't forget anything, do you? Like the time Draco came over and found—"
"Next time I shall let you freeze, imp," Severus retorted, cutting Harry off. That had been embarrassing.
It makes me want to know what happened, desperately, but at the same time I love that you didn't tell us- that Harry and Severus have this history that we don't know about. It makes it much more real.
And I adore the ending! Maybe he'd even teach him to fly. That's just such a lovely line. It says so much about them. : ) I hope he does teach him, and that they live happily ever after.